It is well known in the art that bridges, buildings, overpasses, and other structures are commonly supported by large columns and beams. Regularly these large columns, beams, or other structural members, are constructed from concrete. As concrete members age, however, the structural properties of the members deteriorate. It is extremely expensive, and sometimes impossible, to replace large support members in a structure such as a bridge, without tearing down the entire structure and rebuilding. For these reasons, retrofits have been developed to strengthen aging structural members. The retrofits deliver stresses in the cross-sectional directions of the structural members. Pre-stresses have been found to improve such properties as the shear strength, lateral confinement, and ductility of the structural members.
U.S. Pat. No. 6,247,279 (Murat et al.) discloses one style of retrofit for concrete columns. Murat et al. teach a retrofit system which includes strands which are wrapped around the concrete column, tensioned, and held in place with anchors. Since strands generally can not be threaded, a wedge and anchor system is used to hold the strands in tension around the concrete columns. Since the strands are highly tensioned, they require special equipment, such as a hydraulic jack system, to be installed. Additionally, the hydraulic jack system is required to replace or remove the retrofit, in the event that maintenance must be preformed on either the retrofit or the column. Also, the strands are not flexible enough to bend flush around rectangular columns or other structural members with sharp corners. As a result, this invention requires a plurality of separate corner pieces and risers to effectively be installed on a rectangular column. These additional pieces add to the complexity of installing and uninstalling this retrofit invention in the field, since all of these pieces must be aligned, then held in place while each strand is being installed.
U.S. Pat. No. 6,718,723 (Al-Tuhami) discloses another style of retrofit for strengthening concrete structural elements. The retrofit taught by Al-Tuhami has two separate assemblies which must be custom made at the job site where the retrofit is being installed. First, corner pieces are installed on each of the four corners of a rectangular column. Then a pressure casing is installed over the corner pieces. The pressure casing includes four corner pieces which extend down the desired length of the column at each corner. Alternating pairs of adjacent corner pieces of the casing are bolted together with threaded rods and nuts down the length of the structural member. After the original corner pieces have been tensioned in place by the pressure casing, splices are cut to connect the corner pieces. The splices are welded to adjacent pairs of the original corner members. After a sufficient number of splices have been welded in place, the pressure casing is removed. However, when the pressure casing is removed the splices will undergo a period of elongation. This elongation will loosen the fit of the splices on the concrete element, and therefore lower the pre-stresses supplied by the splices. This invention uses excess material, specifically the pressure casing, which is only attached temporarily to each column. Additionally, there is a substantial amount of fieldwork required, specifically cutting and welding, which is generally more expensive and time consuming than equivalent work done in a machine shop.
Thus, there is a long felt need for a retrofit assembly which pre-stresses structural members and can be quickly and easily installed without the use of special equipment or extensive fieldwork.